review

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Effluents from many food processing industries, slaughterhouses, edible oil processing industries, and dairy product industries contain high lipid. These lipid-rich wastes content lipids as a main ingredient and causes problems during the anaerobic treatment of waste. One of the operational problems associated with lipid is clogging. Besides clogging, it also causes the mass transfer limitation forming a layer on the surface of granules by absorbtion. Due to adhesion of fat, biomass wash out is another problem in anaerobic reactors treating any lipid-based wastewater (Cirne et al., 2007). All these operational problems restrain the efficiency of anaerobic reactors. To overcome these operational problems, generally the lipid content in the waste is removed by physico-chemical pretreatments. However, the higher methane yield is achieved through anaerobic digestion of lipid in comparison with that of proteins or carbohydrates. Hence removal of lipid from waste before anaerobic treatment is not a feasible option in the context of energy recovery from waste (Ahring, 2003). In anaerobic environment, hydrolysis of lipids yields glycerol and free long chain fatty acids (LCFA). In the subsequent steps, glycerol is converted to acetate by acidogenesis. The degradation of LCFA proceeds through the β-oxidation pathway (syntrophic acetogenesis) and either acetate or propionate (in the case of LCFA containing odd-number carbon) along with hydrogen are produced (Weng and Jeris, 1976). Besides operational problem, another issue of concern is inhibitory effect of lipid on anaerobic microorganisms which is primarily caused by LCFA rather than neutral lipid. (Angeladaki et al., 1992). However, lipid hydrolysis or liquefaction of lipid is a ra... ... middle of paper ... ...m and Methanosaetaceae in the system was indicated by cloning and sequencing results. Similarly, Kim and Lee (2013) observed stable archaeal community with dominance of hydrogenotrophic methanogens during repeated batch degradation of cheese processing wastewater. However, bacterial community structure varied significantly. Kundu et al. (2013b) observed stable archaeal community profile while increasing the loading rate of simulated dairy effluent, but a significant decrease in 16S rRNA gene copy number of Methanosaetaceae observed in the reactor demonstrates the sensitivity of this acetoclastic group to the change in loading conditions with lipid-rich wastewater. However, from the VFA profile as high levels of propionate and butyrate were observed, they also suggested that syntrophic acetogenesis might be the rate limiting phenomenon rather than methanogenesis.

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